The skin covering the human body is rich in hair and hair follicles, with over 90% of its surface supporting some type of hair. Biologically defined, hair is the epidermal tissue that is derived from the involutions, called follicles, located in the skin. The bulb region of the hair follicle contains the metabolically active cells of the hair, which derive nutrients from the circulating blood. As such, hair follicle bulb cells are responsive in real-time or near real-time to toxic insults, in much the same way as skin cells.
Chemicals or heavy metals present in the body are brought into contact with the hair follicle by the circulating blood. There, they may enter the follicle and may potentially be incorporated into the hair shaft by the proliferating matrix cells found in the hair bulb. The ability to be incorporated into the hair shaft has led to the possibility of detecting exposure to chemicals and heavy metals via the hair shaft. Termed segmental hair analysis, this has become a popular method to determine exposure to drugs of abuse (e.g. cocaine, amphetamine, methamphetamine, heroin, marijuana, and steroids), therapeutic drugs (e.g. ephedrine, benzodiazepines, and barbiturates), and inorganic heavy metals (e.g. arsenic, lead, and mercury).
Recent reports suggest that long-term exposure to pesticides (e.g. carbamate, organochlorine, and organophosphate (OP)) can also be detected using segmental hair analysis. Others have demonstrated that methomyl, a carbamate pesticide, could be detected in the rabbit hair shaft following a 4-month daily methomyl exposure.
To carry out segmental hair analysis, scalp hair is cut into 1, 2, and 3 cm segments, which approximately corresponds to 1, 2, and 3 month's growth. If scalp hair is not available, other sources of hair can be used for analysis. Segmental analysis can be accomplished with as little as one hair or up to 200 mg of hair may be used to allow confirmation testing. The segments are typically washed to remove external contaminants and the chemicals in the hair are extracted by alkaline digestion, enzymatic treatment, or acid extraction. Several analytical methods are available to determine the concentration, if any, of a particular substance in the hair. Immunoassay, High Performance Liquid Chromatography (HPLC), Capillary Electrophoresis (CE), Gas Chromatography-Mass Spectrometry (GC-MS), tandem mass spectrometry (MS-MS) and Liquid Chromatography-Mass Spectrometry (LC-MS) can be used for analysis. Of these, the most superior in sensitivity, selectivity, and specificity is GC-MS separation and selective ion monitoring (SIM) quantitation. However, tandem MS can be used if the compound of interest is unstable in gas chromatograph separation/analysis.
The main advantage of segmental hair analysis is that successful analysis is possible even months after exposure. Other sample sources for testing, such as urine, offer an immediate window of testing, however, most drugs can only be detected 1 to 3 days following initial use. Although segmental hair analysis is becoming increasingly popular, the risk of a false positive is high when compared to urine or blood testing. Dark hair color, poor personal hygiene, and passive exposure can all lead to false positive segmental hair analysis results.
Due the increased risk of false positives associated with segmental hair analysis and the more invasive collection required for serum and urinalysis, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods of determining levels of toxicant exposure.